1. Field of the Invention
This invention relates to a reduction ratio control mechanism provided within the hydraulic control system of a continuously variable automatic transmission for a vehicle employing a V-belt continuously variable transmission controlled by means of hydraulic servomechanisms, and adapted to control the reduction ratio (the torque ratio) of the V-belt type continuously variable transmission.
2. Description of the Prior Art
A V-belt type continuously variable transmission constitutes, in combination with a forward-reverse drive changing device and a fluid coupling, a continuously variable automatic transmission for a vehicle. This continuously variable automatic transmission, however, will not be able to produce a sufficient torque and hence the vehicle can not start smoothly, when the continuously variable transmission is not set at the maximum reduction ratio at the start of the vehicle. Generally, it is difficult to change the reduction ratio of a V-belt type continuously variable transmission while the operation thereof is interrupted. Accordingly, quick down-shift operation is necessary when the vehicle is stopped suddenly. The principal factors dominating the speed of the down-shift operation are the level of the fluid pressure (the line pressure) applied to the hydraulic servomechanism of the V-belt type continuously variable transmission and the operating speed (the response) of the valves of the reduction ratio control mechanism. The conventional reduction ratio control mechanism includes, as shown in FIG. 34, a reduction ratio control valve 81 disposed between a hydraulic fluid supply passage 2 connected to the hydraulic servomechanism, which receives an increased rate of the hydraulic fluid during down-shift operation (reducing speed) and an reduced rate of the hydraulic fluid during up-shift operation, of the V-belt type continuously variable transmission and a line pressure passage 1 connected to a hydraulic source, for controlling the communication of the hydraulic servomechanism with the hydraulic source or with a drain port, and having a drain port 813 and a drain port 814 for relieving the hydraulic pressure working on the hydraulic servomechanism, a spring 811 for applying a spring force to the spool of the reduction ratio control valve, an up-shift electromagnetic valve 84 and a down-shift electromagnetic valve 85. This reduction ratio control mechanism is adapted to control the down-shift operation by intermittently reducing the pressure applied to the spool, through the operation of the down-shift electromagnetic valve 85 so that the spool moves while compressing the spring 811 to relieve progressively the line pressure held by the closing operation of the up-shift electromagnetic valve through the drain port 813. The spool stops when the external forces working on the spool become equilibrated. Since the spool moves against the spring force of the spring 811 during the down-shift operation, the quick response of the spool is impossible.
Accordingly, the response of the spool can be improved by inverting the respective position of the up-shift electromagnetic valve 84 and the down-shift electromagnetic valve 85 and the respective position of the line pressure passage 1 and the drain port 814 for relieving the hydraulic pressure of the servomechanism of the V-belt continuously variable transmission as shown in FIG. 35, so that the spool will move along the direction of the spring force of the spring during the down-shift operation whereby the moving speed of the spool is increased. According to this method, however, since the hydraulic fluid is drained through the valve port of the down-shift electromagnetic valve 85 and through the drain port 813 at the same time, the line pressure is reduced due to increased draining rate and hence the pressure applied to the hydraulic servomechanism of the V-belt continuously variable transmission is decreased so that the speed of down-shift operation of the V-belt continuously variable transmission is reduced.